1. Field of the Invention
The present invention relates to a sheet stacking apparatus in which sheets are stacked, and an image forming apparatus that includes the sheet stacking apparatus in an apparatus main body thereof.
2. Description of the Related Art
In the past, an image forming apparatus, which forms an image on a sheet, may often include a sheet stacking apparatus, which is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2006-124051, in an apparatus main body thereof. A large number of sheets, on which images have been formed, discharged from an apparatus main body are stacked in the sheet stacking apparatus. Meanwhile, the sheet stacking apparatus disclosed in JP-A No. 2006-124051 is called a stacker that can stack a large number of sheets therein.
FIG. 13 is a schematic front view of the stacker that is disclosed in JP-A No. 2006-124051. The stacker 500 holds a downstream end portion (front end portion) of a sheet in a sheet discharging direction by a gripper 503, pulls the sheet, and stacks the sheet on a sheet stacking table 505. That is, after receiving the sheet discharged from an apparatus main body of an image forming apparatus by an inlet roller 501, the stacker 500 grips the front end portion of the sheet by the gripper 503, pulls the sheet, and makes the sheet collide with a stopper 504.
When colliding with the stopper 504, the sheet falls down from the gripper 503 and is stacked on the sheet stacking table 505. The stacker 500 repeatedly operates until a predetermined number of sheets are stacked on the sheet stacking table 505. Members 506 and 507 are front and rear end pressing members for pressing the discharged preceding sheet so that the discharge of the succeeding sheet is not hindered.
In a sheet stacking apparatus (hereinafter, referred to as a stacker) disclosed in U.S. Pat. No. 6,641,133, a gripper, which grips a front end portion of a sheet, is opened by being abutted against a fixing member so that a sheet is released.
However, the stackers disclosed in JP-A No. 2006-124051 and U.S. Pat. No. 6,641,133 separate and stack a sheet by making the gripper be bumped against the stopper and the fixing member. For this reason, sheets, which are to be stacked on the stackers disclosed in JP-A No. 2006-124051 and U.S. Pat. No. 6,641,133, do not have horizontal components of velocity when being moved by the gripper, and freely fall down on the stackers under one's own weight. Since the fall times of the sheets are lengthened due to air resistance, it is necessary to start discharging a subsequent sheet after completion of stacking a preceding sheet and the fall positions of the sheets become non-uniform due to air resistance when the sheets fall down, the stackers in the related art have low stacking efficiency and poor stack alignment of sheets. Accordingly, it is considered to shorten the fall time of a sheet and make the fall position of a sheet be uniform by pressing a falling sheet from above. However, if a mechanism for pressing a falling sheet from above is provided in the stacker, there are other problems in that the stacker is complicated and increased in size.
Further, in the stacker disclosed in JP-A No. 2006-124051, the gripper is bumped against the stopper while holding a front end portion of a sheet, so that the sheet is separated from the gripper. Since the front end portion of the sheet is restricted by the gripper, there has been a concern that the stopper damages the front end portion of the sheet when stopping the inertial force of the gripper. In particular, there has been a concern that the stopper damages the sheet fed from the apparatus main body of the image forming apparatus at high velocity.
Further, in the stacker disclosed in U.S. Pat. No. 6,641,133, a mechanism for separating a sheet from the gripper is complicated, so that the stacker is increased in size and manufacturing cost thereof is increased.
When there are damaged sheets in an image forming apparatus including a sheet stacking apparatus, which has a concern that the front end portion of the sheet is damaged, in an apparatus main body thereof, the image forming apparatus should form images several times as many as the number of damaged sheets. For this reason, the productivity of the image forming apparatus is low.
The invention provides a sheet stacking apparatus that releases the holding of a sheet without bumping the sheet against a stopper, quickly and stably discharges and stacks the sheet, and improves stacking efficiency and stack alignment without increasing the apparatus in size.
The invention provides an image forming apparatus that includes a sheet stacking apparatus decreasing the damage to a sheet and improves productivity.